The Race of Racers

Snakes are, for the most part, fairly retiring animals, little seen even in areas where they may be abundant. In much of North America, however, one of the most commonly encountered snake species is the racer Coluber constrictor. This moderately large non-venomous snake, with the largest individuals approaching two metres in length, is a widespread inhabitant of open habitats such fields, brushland or open woodlands. Its distribution is centred over much of the continental United States, being found in most regions except much of the arid south-west. Outside the United States, it has a very patchy distribution in southernmost Canada, Mexico and northern Central America. Most recent authors treat it as the sole species in the genus Coluber; other species historically assigned to this genus from across the Holarctic region now being treated as separate. These include the North American whip snakes of the genus Masticophis, believed to the closest relatives of the racer (Myers et al. 2017).

Southern black racer Coluber constrictor priapus, copyright Peter Paplanus.

Adult racers are generally uniformly dark in coloration dorsally, with a lighter-coloured venter, though juveniles have a blotchy checkered pattern (Fitch 1963). The exact shade varies across the species' range and a number of subspecies have been recognised such as the blue racer Coluber constrictor foxii and the northern black racer C. c. constrictor. In general, individuals are darker towards the east and north, and lighter towards the west and south. Wilson (1978) listed eleven subspecies of C. constrictor whereas a phylogeographic study of the species by Burbrink et al. (2008) identified six major lineages. As well as coloration, members of these lineages may differ in factors such as behaviour or genital morphology, and future studies may see them elevated to the rank of separate species.

Blue racer Coluber constrictor foxii, copyright Peter Paplanus.

The natural history of Coluber constrictor was reviewed in detail by Fitch (1963). As the vernacular name of 'racer' suggests, Coluber constrictor is a fast mover. Its diet contains a mixture of small vertebrates, such as frogs, lizards and small mammals, and large invertebrates such as grasshoppers, crickets and caterpillars. Foraging individuals often hold the front end of the body raised above the ground. Despite their species name, racers do not kill their prey by constriction. Instead, they mostly capture prey by darting forward quickly and grabbing it, often swallowing prey live. Fitch recorded one occasion when he observed a racer in the process of subduing a large skink. While the snake was swallowing its prey, Fitch attempted to capture it. The racer disgorged the skink, and both snake and lizard escaped the scene. Diet may vary with size, with smaller individuals taking a higher proportion of invertebrates, but also varies with range. Populations in the west may primarily feed on insects whereas others may almost exclusively take vertebrates. The northern black racer of the northeastern United States is the most inclined of the subspecies to feed on other snakes. Cannibalism is not unknown; at least one author recorded observing it among broods raised in captivity. In one case, two young racers latched onto a single lizard. One of them successfully downed the lizard, and then also continued on to devour the snake attached to the other end, despite the swallowed snake being nearly as large as its swallower.

Eastern yellow-bellied racer Coluber constrictor flaviventris, copyright David Sledge.

During winter, racers hibernate in crevices and hollows among rocks. Preferred hibernation locations are often at the top of hills, away from their usual hunting sites. Mating and egg-laying occurs shortly after emergence with the peak of egg-laying being in early June (Rosen 1991). Racers, particularly the large northern black, may become more aggressive during this period. Eggs are buried shallowly, in loose soil or under litter, though females may take advantage of abandoned mammal burrows to provide a more secure location. As with other snakes, laying seems to be a matter of pump and dump; I didn't come across any references to females protecting clutches. After hatching, males take about a year to reach sexual maturity whereas the larger females take about two years. Fitch (1963) reports encountering the same individuals over the course of several years (recognisable by their bearing the scars of prior collection of scale samples). Nevertheless, the majority of racer hatchlings do not survive their first summer. Few get the opportunity to seek out shelter for their winter's sleep.

REFERENCES

Burbrink, F. T., F. Fontanella, R. A. Pyron, T. J. Guiher & C. Jimenez. 2008. Phylogeography across a continent: the evolutionary and demographic history of the North American racer (Serpentes: Colubridae: Coluber constrictor). Molecular Phylogenetics and Evolution 47: 274–288.

Fitch, H. S. 1963. Natural history of the racer Coluber constrictor. University of Kansas Publications, Museum of Natural History 15 (8): 351–468.

Myers, E. A., J. L. Burgoon, J. M. Ray, J. E. Martínez-Gómez, N. Matías-Ferrer, D. G. Mulcahy & F. T. Burbrink. 2017. Coalescent species tree inference of Coluber and Masticophis. Copeia 105 (4): 640–648.

Wilson, L. D. 1978. Coluber constrictor Linnaeus. Catalogue of American Amphibians and Reptiles 218: 1–4.